1. Field of the Invention
The present invention is directed to a method for seeking the center of a recorded data track on a tape with an accuracy which is not limited by the stepper mechanism resolution of the head positioning apparatus and is also not limited by tape wander, thereby permitting data tracks to be recorded on a tape with higher track density.
2. Description of the Prior Art
One factor limiting the data track density (i.e., the closeness of adjacent data tracks) on a magnetic tape is the ability of the drive system to reliably position a write or read head relative to a selected data track, so that data is accurately written on, or read from, the correct data track. Head positioning systems are known which use a so-called "edge seek" technique, wherein a read head is moved perpendicularly to a data track, i.e., toward an edge thereof, and when an output from the read head exceeds a predetermined threshold, the read head is assumed to be over the track. At least two accuracy-limiting factors exist in the use of this known technique. First, the read head is not moved in a continuous path toward the tape edge, but is moved in steps by a head stepping mechanism. The accuracy with which the position of the tape edge can be determined is, therefore, limited by the size of the step with which the read head is moved. Another source of error is tape wander error. This results from the continuous movement of the tape in a direction perpendicular to the direction of tape transport, which results from unavoidable, slight wobbling of the drive reel and the take-up reel and the tape capstans. Since these components must be mounted so as to permit rotation thereof, there is always some unavoidable play in the bearings of those components, causing such wobble. This results in the tape exhibiting a slight undulating motion relative to any stationary point of reference. This also results in so-called tape slope error, in systems in which separate read and write heads are used. The read and write heads must necessarily be spaced a slight distance from each other, with their respective centers being aligned on a straight line. Due to the slight slope of the tape relative to this straight line, caused by the undulating path of the tape due to tape wander, even if the read head accurately identifies the location of the track edge, the track edge will not be precisely at that position when it reaches the write head since the tape is canted at a slight angle relative to the straight line between the centers of the write and read heads.
Given a finite tape width (i.e., the extent of the tape in the direction perpendicular to the direction of tape transport), it is desirable to pack as much data therein as possible, which means having as many data tracks as possible within that width. The number of data tracks for a given tape width is referred to as the track density. The ability to achieve higher track densities is directly related to the ability to accurately identify the location of a data track, both for writing and reading data. Regardless of the density with which data can be written on a tape, if a desired track cannot be identified and reliably read by the read head as the tape is moving past the read head, the data cannot be accurately retrieved.
As noted above, the search algorithm known in the prior art typically searches for the lower edge and/or the upper edge of a selected data track by monitoring for which step position the output from the read head is above or below a specified threshold. The threshold is typically chosen so that 10% to 50% of the read head is over the track being read when the threshold is reached. In systems wherein both the lower and upper edges of the data track are found, the center of the data track is then calculated as the position midway between the lower and upper edge positions. The accuracy of such systems has, therefore, not been significantly affected by the quality of the read output from the track, but has been limited by the resolution of the stepping mechanism (the size of each step) and the dynamic vertical (perpendicular) movements of the tape (tape wander) during the seeking operation. The accuracy of position identification has been limited by the stepping resolution because, when comparing the read head output with the fixed threshold in a threshold comparator, it is not possible to determine whether the output at the preceding step was actually closer to the threshold level than the step which actually puts the output over the threshold level. For example, the read head could be positioned just short of the edge of a selected data track and generate an output which would be below the threshold, and then when moved by the finite step of the stepper mechanism, the next position of the head can cause the threshold to be significantly exceeded. It is this step which would be interpreted as corresponding to the edge position, whereas the read head would have actually been positioned closer to the edge at the preceding step.
Tape wander also has contributed to error, because it has heretofore not been possible to measure the average output of the read head, and thereby averaging the tape wander and taking it into account. This has resulted, in known systems, in the lower and upper track edges being identified as the extreme uppermost or extreme lowermost positions of the track in its undulating path.
In conventional systems, the stepping resolution has typically been in the range of 5 to 10 .mu.m (0.196 to 0.394 mil), and tape wander is specified to be a maximum of .+-.12.7 .mu.m (.+-.0.5 mil).